Laser scanning involves moving a laser beam along a surface to be scanned and can be used for both writing and reading purposes. For example, laser scanning is used for writing in printing systems, where the scanned beam activates spots on a printing medium, and in cutting systems where the scanned beam cuts material. For reading, laser scanning is used in inspection systems and in copiers which use the scanned beam to illuminate consecutive spots of a surface to be viewed or a page to be copied.
FIG. 1, to which reference is now made, schematically illustrates a laser scanning system for printing and a surface 10 of a medium to be activated. The system includes a laser 12, a pre-scan optical system 16, a scan unit 14, and a post-scan optical system 20. The scan unit 14 can be an acousto-optic deflector, a polygon deflector a hologon deflector or an oscillating mirror.
The laser 12 produces a beam 22, the pre-scan optical system 16 provides the scanned beam with the desired optical properties and the scan unit 14 deflects the beam 22 to provide the scanning motion, as indicated by arrow 24. The post-scan optical system 20 focuses the scanned beam on the medium 10, thereby to produce the printing spot, and converts the angular scan of arrow 24 to a linear scan, as indicated by arrow 26.
Due to the action of the scan unit 14, the focused beam scans a portion of the medium 10, as indicated by arrow 26, in one direction, known as the "fast scan direction". The medium 10 typically is moved, as indicated by arrow 26, in a second direction, orthogonal to the fast scan direction. This is generally known as the slow scan direction. The fast and slow scan directions together provide two-dimensional scanning. Alternatively, the scan unit 14 can produce two-dimensional scanning if it includes means for deflecting the beam along a second direction.
The scanning rate (defined as pixels/sec or spots/sec) of any laser scanning system is a function of the velocity of the spot and the size of the spot, both of which are functions of the limitations of the scan unit. The scanning rate is thus limited by the fundamental parameters and quality of the scan unit. It will be appreciated that, for a given pixel or spot size, the scanning rate determines the throughput (e.g. number of pages printed or number of wafers inspected within a given period of time).
It is known to increase the throughput of a laser scanning system for printing by increasing the number of beams being scanned at one time. One such system, with 32 beams, is the ALTA-3500, commercially available from Etec Systems Inc. of California, USA.
FIG. 2, to which reference is now briefly made, schematically shows the system, but with only three beams 30. The multiple beams can be aligned along the fast scan direction, as shown, or along the slow scan direction. A beam generating unit 32, such as multiple lasers or a single laser with multiple beam splitters, produces the multiple beams 30. The multiple beams 30 pass through a system similar to that shown hereinabove for FIG. 1 but whose elements are designed for multiple beams. Thus, the scan unit and pre- and post-optical systems carry similar reference numerals as those of the scanning system of FIG. 1 but are additionally marked with an apostrophe (').
The multiple processed beams, labeled 34, are scanned along the surface of the medium 10, thereby generating multiple parallel scan lines at one time. This typically increases the throughput of the scanning system by the number N of beams used, where an N of two to many hundreds are known.
Laser scanning systems for inspection systems utilize the scanned light for illumination of an article to be inspected by one or more detectors. Such a system is shown schematically in FIG. 3, to which reference is now made. Like the previous scanning systems, it also includes laser 12, scan unit 14, pre-scan optical system 16 and post-scan optical system 20. However, the inspection system also includes multiple light detectors 40 for detecting the shape of features on a surface 42, such as the surface of a semiconductor wafer, from different viewing perspectives. The movement of the surface 42 is indicated by arrow 44.
The scanning elements illuminate the surface 42 from above and the surface 42 scatters the light in many directions, as a function of the optical characteristics of the features thereon. The inspection system of FIG. 3 is a "dark field" inspection system since its detectors 40 collect the light scattered from the surface 42 at an oblique angle .beta. which is outside of the convergence angle of the post-scan optical system 20.
The oblique angle .beta. varies depending on the type of surface to be inspected and the type of features to be inspected. The light detectors 40 are typically non-imaging detectors, such as a photomultiplier tubes, which measure the changing intensity, over time, of the light impinging upon them. As is known to those skilled in the art, in order to differentiate the light from different pixels on the surface 42, the signal from the photomultiplier tube must be sampled at a rate corresponding to the spot size and to the velocity of the spot on the surface 42. This may be called "temporal resolution".
As in the other scanning systems, the scanning rate of the inspection system of FIG. 3 is a function of the fundamental parameters and the quality of the scan unit 14. Of course, as in other scanning systems, it is desirable to increase the scanning rate of the inspection system. However, an inspection system does not easily lend itself to operating with multiple beams. One reason is that non-imaging detectors do not discern the position from which the light was scattered. Adding other beams would, therefore, cause cross-talk on the detectors caused by the signals from the other spots. Imaging detectors cannot easily be incorporated into a dark field imaging system since, due to oblique incidence angle .beta., the collection optics cannot resolve sufficiently small pixels such as is possible with detectors placed at a non-oblique angle.